U.S. Pat. No. 4,817,828 discloses an inflator which comprises a source of gas for inflating an inflatable vehicle occupant restraint such as an air bag. The inflator contains a body of ignitable gas generating material. When the vehicle experiences deceleration of at least a predetermined amount which indicates the occurrence of a vehicle collision, the gas generating material in the inflator is ignited. As the gas generating material burns, it generates a large volume of gas which is directed to flow from the inflator into the air bag to inflate the air bag. When the air bag is inflated, it extends into the vehicle occupant compartment to restrain an occupant of the vehicle from forcefully striking parts of the vehicle.
The inflator disclosed in the '828 patent has a cylindrical housing. The housing includes a tubular inner wall and a tubular outer wall. A cylindrical filter is contained in the housing concentrically between the tubular inner wall and the tubular outer wall. The tubular inner wall defines a cylindrical combustion chamber in which the body of gas generating material is contained. An igniter for igniting the body of gas generating material is located at one end of the combustion chamber. A plurality of gas flow openings extend through the tubular inner wall, and thus direct the gas to flow radially outward from the combustion chamber and through the filter toward the tubular outer wall. A plurality of gas flow openings extending through the tubular outer wall similarly direct the gas to flow radially outward from the housing toward the air bag.
The body of gas generating material has substantially the same size and shape as the cylindrical combustion chamber defined by the tubular inner wall of the housing. The body of gas generating material thus has an elongated cylindrical shape with longitudinally opposite ends that are located adjacent to respective opposite ends of the combustion chamber. Moreover, the body of gas generating material is defined by a plurality of separate grains of gas generating material. The grains of gas generating material are arranged concentrically in a row which extends along the length of the combustion chamber. The igniter, which is located at one end of the combustion chamber, is thus located adjacent to the first few grains of gas generating material in the row. As a result of this arrangement, ignition of the body of gas generating material is initiated at the first few grains of gas generating material in the row, and proceeds progressively along the length of the body of gas generating material as successive grains in the row are ignited.
In order to maximize the rate at which ignition progresses along the length of the body of gas generating material, the surface of each grain of gas generating material is coated with a material that is more readily ignitable than the gas generating material of which the grains are formed. The body of gas generating material is thus provided with a coating of an ignition enhancing material which maximizes the rate at which ignition proceeds over the surface of the body of gas generating material. However, such a coating is known to be expensive and difficult to apply.